In recent years, digital projection systems using spatial light valve modulators, including digital micro-mirror devices (DMDs) and transmissive liquid crystal displays, have been receiving much attention. These types of systems provide a high standard of display performance including high resolution, high brightness and a high contrast ratio. For example, a conventional DMD projection display system includes at least a light source, usually a high-pressure discharge lamp, a color wheel, a DMD, and an optical system. Most consumer DMD projectors employ color wheels to filter the incoming white light and provide sequential color light for illuminating the DMD panel. The color wheel devices are described in a numerous patents such as U.S. Pat. Nos. 5,680,180, 6,591,022, 6,767,100 and 6,830,343 which are herein incorporated by reference. One drawback of these types of prior art systems is that the color gamut of the projection system, with color wheel, is often limited due to the spectrum distribution of the white light source. Another inherent disadvantage is that the motor for driving the color wheel can be a source of ambient noise.
Additionally, short arc type discharge lamps such as mercury lamps, metal halide lamps and xenon lamps are widely used as the light source for the projection type display apparatus. However, there are some drawbacks with these types of short arc type discharge lamps. One of these is that all of the light except blue, green, and red will be filtered out resulting in a loss in efficiency. In addition, a fly-eye type of integrator or a light pipe optical integrator is required for collecting, homogenizing and guiding light from the light source. This has the effect of making the system very bulky and complex. Furthermore, there is a significant amount of ultraviolet (UV) and infrared (IR) light emitted from these lamps. The unfiltered UV light can work to reduce the operating lifetime of both the optical components and display panel in the system. The IR light can require means for providing additional cooling. To eliminate UV and IR light emitted from the lamp, UV/IR filters are generally employed that result in an increase in system complexity. Therefore, to avoid all aforementioned drawbacks of the previous illumination systems, it is desirable to devise an illumination apparatus that has an enhanced efficiency, prolonged lifetime, compact size and low cost to replace the short arc type discharge lamps used in current products.
Significant efforts have been dedicated towards utilizing light-emitting diodes (LEDs) in projection illumination systems as a light source. LEDs show great promise since they consume less power, release less heat, and can have a longer life time. The LEDs offer high light efficiency since all the spectrum of red, green and blue light from LEDs can be utilized. In addition, LEDs with three primary colors can produce a wider color gamut than conventional white lamps. An example of an LED light source having uniform light distribution can be found in U.S. Pat. No. 5,420,444.
Those skilled in the art will further recognize that light is not easily diffused using an LED-type light source and the output lumen power of an LED is generally much less than a short arc type discharge lamp. In practice, it is often a challenge to get a good uniform light beam from the LED source with the least amount of optical loss. Optical loss is typically caused by a fly-eye lens array or waveguides integrated into the projector illumination system. The prior art system as disclosed in U.S. Pat. No. 6,224,216 describes an embodiment of a triple path projector employing three single color LED arrays. These LED arrays emit light propagating along separated paths through fiber bundles to respective separated light pipe integrators and respective display devices. One disadvantage in this type of implementation is that the coupling between LEDs and fibers is often a problem. Practically, due to coupling and transmitting losses, it is difficult to efficiently couple light emitting from LED arrays to corresponding fiber bundles and light pipes.
In the invention described in U.S. Pat. No. 6,220,714, a projection system using LEDs for illumination is disclosed where light emitting from red, green and blue LEDs or LED arrays is collimated by condenser lenses. The light then propagates through fly-eye type of integrators for illuminating a single panel. Based on the geometry of the integrator lens, only the surface area of light-emitting region within a certain field of view can be effectively used for illuminating the panel. A similar system can be found in U.S. Pat. No. 6,644,814 that describes an LED-illumination-type DMD projector with one panel. Three sets of LED arrays are employed as light sources where three first fly-eye lenses are disposed in contact with respective red, green or blue LED arrays. The exit pupil of each element of the fly-eye lenses is illuminated by the LED array. A second fly-eye lens then forms images of the illuminated pupils of the first fly-eye lens and makes them overlapped on the DMD panel. The main problem with this system is that some light from the LEDs cannot enter the corresponding lens of the first and second fly-eye lenses due to aberration and aperture limitation of lens arrays. Thus, a portion of illumination light falls outside of the panel area resulting in low light efficiency and low contrast.
Accordingly, it has proven to be a challenge to provide an LED optical system for DMD projector that overcomes the shortcomings of the existing prior art LED illumination systems. The need exists to provide an optical light engine using LEDs that eliminate the use of a traditional homogenizer such as fly-eye lens arrays and integrator waveguides to avoid considerable optical loss. Furthermore, there is a need to provide an LED optical light engine system that offers advantages in compactness, simplicity and low cost.